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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 20 (2009) 405702 (7pp) doi:10.1088/0957-4484/20/40/405702

High volume fraction carbonnanotubeepoxy compositesZ Spitalsky 1, 4, G Tsoukleri 1, D Tasis 2 , C Krontiras 3, S N Georga 3

and C Galiotis 1,2,5

1 Foundation for Research and Technology Hellas, Institute of Chemical Engineering and HighTemperature Chemical Processes (FORTHICE/HT), 26504 Patras, Greece2 Department of Materials Science, University of Patras, 26504 Rio Patras, Greece3 Department of Physics, University of Patras, 26504 Rio Patras, Greece

E-mail: [email protected]

Received 16 June 2009, in nal form 17 August 2009

Published 8 September 2009Online at stacks.iop.org/Nano/20/405702

AbstractA versatile processing technique for fabricating epoxy nanocomposites with a high weightfraction of oxidized multi-walled carbon nanotubes is presented. Thin carbon nanotube basedpreforms were prepared through an oxidationltration protocol and then immersed in apre-polymerized epoxy/curing agent solution in acetone. By adjusting the conditions for theoxidation of carbon nanotubes and the epoxy concentration in the as-prepared solution, highloading of graphitic nanostructures was obtained. Tensile tests indicated that the elasticmodulus and strength of certain composites prepared by in situ polymerization as above wereimproved by 100% and 60%, respectively, compared to neat epoxy. In addition, the compositesheets showed comparable electrical conductivity values to the neat carbon nanotube paper.These results suggest that targeted chemical modication of the carbon nanotube surface is aneffective way to enhance the electrical and mechanical properties of carbon nanotubepolymercomposites.

1. Introduction

Since the landmark paper of Iijima in 1991 [ 1], carbonnanotubes (CNTs) have been the focus of considerableresearch due to their excellent mechanical, electromagneticand chemical performance. Lightweight multifunctionalcomposites with enhanced properties can potentially beproduced by effectively incorporating individual CNTs intopolymer matrices [ 2, 3]. Composites produced by conventionalmethods, such as solution mixing and melt blending, usuallyfail to yield signicant improvements of bulk mechanical andelectrical properties, when compared to individual nanotubes.

The main reasons for the poor properties of compositesare the inadequate weight fraction of CNT material (inmost cases below 5%), non-uniform CNT dispersion withinthe polymer matrix and the lack of adhesion between theconstituents of the composite. Recently, several innovativeprocessing techniques have been developed in order to

4 On leave from: Polymer Institute, Slovak Academy of Sciences, Bratislava,Slovak Republic.5 Author to whom any correspondence should be addressed.

overcome the above obstacles. Macroscopic forms of entangled CNTs, such as microbres [4, 5], aligned arrays [6]and membranes [ 716], have been used as scaffolds forthe fabrication of multifunctional composites by polymerimpregnation into the free pores of the CNT network. Inparticular, CNT/polymer thin lms have attracted increasedinterest in many applications due to their excellent electricalproperties [16, 17]. Signicant enhancement of the structuralintegrity of the lamellar composites has been reported mainlyfor the elastic modulus values, whereas tensile strength is notimproved appreciably in most cases [ 13, 14, 18].

Different approaches to infuse polymer chains within thevoids of a CNT membrane include vacuum-aided solutioninltration, in situ polymerization, hot compression andsoaking of CNT sheets in polymer solution [ 616]. Inmost cases, these processes involve the fabrication of CNT-based composites with thermoplastic matrices. Inaddition, most CNT membranes are made of single-walledtubes (SWCNTs), which are more expensive than multi-walled derivatives (MWCNTs) and difcult to exfoliateindividually. MWCNTs, instead, are less agglomerated

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Nanotechnology 20 (2009) 405702 Z Spitalsky et al

and can form paper-like networks with homogeneousporosity. Low cost MWCNT/epoxy composites can serve asmultifunctional materials with light weight, high strength andhigh electrical conductivity for many industrial applications,such as components in the aircraft and electronic industries.Depending on the processing steps during the fabrication

of such composites, it is possible to tailor their mechanicalproperties, such as elastic modulus and strength, as well astheir electrical conductivity.

This study is a continuationof previous works of our groupregarding the effect of chemical oxidation on the structuralintegrityof MWCNT material and the preparation of functionalCNT buckypapers by a dispersionltration protocol [1921].These buckypapers, composed of randomly oriented tubes withtailored chemical environment, can be used as a preform forfabricating CNT/epoxy composites by just immersing the CNTlm in a diluted solution of freshly made pre-polymerizedepoxy/curing agent mixture. By this approach, MWCNT-

based nanocomposites with a high weight fraction ( > 10%)can be prepared. The effects of the epoxy concentrationand the degree of CNT oxidation on the nal morphologyof the composites, as well as on their mechanical/electricalproperties, were investigated in detail.

2. Experimental details

2.1. Materials

The MWCNT material employed in this work was suppliedby Nanocyl (Belgium). The CNTs were synthesized bycatalytic carbon vapour deposition and had a purity of around 80%. The CNT diameter ranged between 10and 20 nm. Chemical oxidation/purication of MWCNTswas carried out by various reagents, such as piranhasolution (a mixture of hydrogen peroxide and sulfuric acidH2O2 /H2SO4), potassium permanganate/sulfuric acid solution(KMnO 4 /H2SO4) and hot concentrated nitric acid (HNO 3),as described previously [ 19, 22]. The average length of theresultingoxidized CNTs was estimated by electron microscopyimaging as about 700 nm [ 19]. Epikote 828 (a mediumviscosity liquid epoxy monomer produced from bisphenolA and epichlorohydrin) and Epikure F205 (a low viscosity,modied cyclo-aliphatic amine curing agent) were purchasedfrom HEXION Specialty Chemicals, USA.

2.2. Buckypaper fabrication procedure

Concerning the fabrication of MWCNT buckypapers, stableaqueous CNT suspensions at a concentration of 1 mg ml 1

were prepared by tip sonication for 60 min. These dispersionswere then vacuum ltered through a polycarbonate membranelters of pore size 450 nm. After drying at room temperaturein a vacuum oven for 24 h, the CNT lms were peeled off from the ltration membrane. The average thickness of theproduced buckypapers was approximately 200 m and theirdiameter was about 9 cm.

2.3. Soaking of CNT buckypapers

Soaking solutions were prepared by dissolving an epoxymonomer/curing agent mixture (stoichiometric weight ratio100:58) in acetone. The as-prepared solutions contained 1,5, 10, 15, 20, 25, 30, 40, 50, 60 and 70 w / w% of epoxycomponents in 40 ml of acetone, respectively. The MWCNTbuckypaper was immersed into the soaking solution for aperiod of about 1 h. Subsequently, the soaked buckypaper wasremoved from the acetone bath, left to dry for 1 h and thencured at 110 C overnight.

Following the same procedure, samples of HNO 3- andKMnO 4-treated buckypapers were also used for the fabricationof epoxy composites. As a parallel experiment, an acetonesolution of epoxy constituents was ltered through (rather thansoaking) the CNT buckypaper along its thickness directionunder vacuum conditions, following the work of Wang etal [8].Filtration of the acetone solution was repeated several timesby using the same ltrate in every cycle. The impregnated

sample was cured under the same condition as the abovementioned soaked samples. Finally for comparison purposes,a neat epoxy resin sample was prepared by casting a 30 w / w%acetone solution of epoxy monomer/curing agent mixture andcured under the same condition as the composites.

2.4. Characterization

Scanning electron microscopy (SEM) was performed usinga LEO SUPRA 35 VP scanning electron microscope.All specimens were sputtered with gold before imaging.Mechanical testing was performed in a TA InstrumentsDynamic Mechanical Analyzer Q800 with a strain rate of 0.02 min 1 on strips of dimensions 20 mm 4 mm. Foreach sample, stressstrain curves of ve different strips wererecorded. The maximum force that DMA Q800 can sustain is18 N. For composite specimens that reached that force limitin tension, a mechanical frame (Hounseld H25KM) with aload cell of 2000 N was employed to record their failurestress/strain characteristics. For consistency, all measurementswere performed on strips of the same dimensions and at thesame strain rate.

Thermogravimetric studies were performed on a TGA2950(TA Instruments) under a nitrogen atmosphere at a heatingrate of 10 C min 1 in temperature range from 30 to 650 C.

The electrical conductivity measurements were performed us-ing the well established van der Pauw method [ 23]. Measure-ments were taken as a function of temperature from 100 to+ 150 C in steps of every 10 C under an inert N 2 atmosphere.

3. Results and discussion

3.1. Morphology

Recently, MWCNT buckypapers have been prepared bya sonication-assisted exfoliation/ltration protocol [ 20, 21].Mechanical measurements have shown that the average tensilestrength and elastic modulus of dry buckypapers can reachvalues as high as 4 MPa and 2 GPa, respectively [ 21]. Asmentioned earlier, in order to optimize the stress transfer to

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(a) (b)

(c) (d)

(e)

Figure 1. SEM images of cross-section area for: (a) neat piranha buckypaper; (b) a sample of composite from piranha-treated buckypaper by5 w/ w% epoxy solution with negligible infusion of epoxy; (c) a sample of composite from piranha-treated buckypaper soaked in 25 w / w%epoxy solution with partial and inhomogeneous penetration of epoxy; (d) a sample of piranha composite from soaking in 60 w / w% epoxysolution with homogeneous penetrationcross-section on the edge; and (e) sample of composite with large voids from KMnO 4-treatedbuckypaper in 50 w / w% epoxy solution.(This gure is in colour only in the electronic version)

the CNT reinforcement during mechanical loading it is of vital importance to impregnate the dry MWCNT lms withan epoxy matrix. However, this cannot be accomplishedby conventional impregnating techniques using neat resin athigh temperatures as the epoxy is generally too viscous topenetrate and therefore adequately wet the nanotubes. Inthis work, the impregnated composite lms were fabricatedby immersing/soaking the CNT sheets into a freshly mademixture of pre-polymerized epoxy/curing agent. Since variousconcentrations of epoxy resin in the soaking solutions mayresult in different CNT weight fractions in the compositelms, we prepared soaking solutions with 11 different epoxyconcentrations (from 1 up to 70 w / w% epoxy content).

The morphology of all composites was examined bySEM. Figures 1(a)(d) show images of neat buckypaper made

from piranha-treated MWCNTs and its corresponding epoxycomposite. Soaking of buckypapers with a low concentrationof epoxy components in acetone caused inhomogeneouswetting of the nanotubes, as shown by the presence of randomly dispersed islands of resin on the CNT surface(gures 1(b) and (c)). The threshold epoxy concentrationin acetone for obtaining fully homogeneous composites was30 w/ w% whereas the upper limit of epoxy concentration forcomposite production was estimated to be 70 w / w%. At thatlevel of concentration, the viscosity of the acetone solution wasquite high and, therefore, it was necessary to use an externalforce in order to immerse and hold the buckypaper insidethe solution. As seen in gure 1(d), the homogeneity of theresin impregnation is indeed remarkable and also extends tothe edges of the specimen. Based on these observations, we

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Table 1. Thickness ( D , m) of CNT buckypapers and theircorresponding composites. In the last two columns, the CNT contentof various buckypaper samples is shown.

Sample

Epoxy concentrationin soaking solution(%)

D( m)a

CNTcontent(%)b

CNTcontent(%)c

Piranha buckypaper 0 205 100 96.3Piranha composite 30 180 27.9 31.3Piranha composite 40 120 27.3 17.1Piranha composite 50 150 20.1 10.1Piranha composite 60 240 12.5 8.6Piranha composite 70 390 9.8 7.3Filtration30 30 230 14.9KMnO 4 composite 50 49.4 49.2KMnO 4 composite 60 37.8 35.2HNO3 composite 50 29.6 27.4HNO3 composite 60 21.3 16.7

a Determined by SEM.b Ratio of buckypaper network weights before soaking and aftercuring ( 100).c Determined from TGA at 550

C.

conclude that the ideal resin concentration for the preparationof homogeneous epoxy composites from piranha-treated CNTbuckypapers should be in the range of 3070 w / w%.

The thickness of neat buckypapers was about 200 mas determined by SEM analysis; the thickness and thecorresponding estimated CNT weight fraction of the variouscomposites are summarized in table 1. As expected, the CNTcontent is decreasing with increasing epoxy concentrationin the soaking solution. Buckypapers were swollen after

immersion in acetone solution but they shrank during curingof epoxy within the CNT network. At relatively lowconcentrations, the thickness of the nal composite decreasedto almost a half of the initial thickness of the neat buckypaper.However, when the whole volume of pores inside buckypaperis lled with resin (concentration 60 or 70 w / w%), swelling of composite prevails over shrinkage of the polymer network andthe thickness of the impregnated composite was twice that of dry buckypaper.

3.2. Conductivity measurements

The specic electrical conductivity measurements of the com-posites prepared from piranha-treated buckypapers were per-formed using the well established van der Pauw method [ 23].Measurements were performed as a function of temperaturefrom 100 to + 150 C in steps of 10 C under an inert nitro-gen atmosphere. Figure 2 presents the experimental results of specic electrical conductivity as a function of reciprocaltemperature for buckypaper composites soaked in 30, 40, 50,60 and 70% acetone solution. In table 2, representative valuesof specic electrical conductivity are given at three differenttemperatures. For comparison, the value of for the neat buck-ypaper is also included at + 20 C. As expected, buckypaperepoxy composites exhibit lower conductivity compared to thatof the neat buckypaper [ 13]. It is also evident that the spe-cic electrical conductivity decreases with increasing epoxy

Figure 2. Temperature dependence of specic electrical conductivity for piranha-treated buckypapers soaked by epoxy solutions of different concentrations.

resin content (gure 2). For the sample with the lowest con-ductivity, this value is over three orders of magnitude higherthan the value of epoxy composites with low volume fractionof piranha-treated MWCNTs [24].

Regarding the evolution of the specic electricalconductivity two distinct temperature regions are present(gure 2): the low temperature region ( 100 to + 80 C) where is found to approach the asymptote exhibiting a very weak temperature dependence and the high temperature region ( + 90to + 150 C), where the specic electrical conductivity evolvesmore rapidly with temperature. In both temperature regions, follows an Arrhenius behaviour as shown by the solid lines ingure 2. In the low temperature region, an activation energyof 6 meV was found, while in the high temperature region, thecalculated activation energy is 0.18 eV. This overall behaviourof the specic electrical conductivity has also been observedfor neat buckypapers in some recent studies [ 25, 26].

Different electrical conductivity models such as uctua-tion-assisted tunnelling or variable-range hopping have beeninvoked in order to explain the electrical conductivity of CNT buckypapers. It is well known that mats consistingof MWCNTs are a mixture of metallic and semiconductingnanotubes exhibiting mixed temperature dependence of theirelectrical conductivity [ 25]. As mentioned earlier, our

results are better tted by an Arrhenius model instead of the uctuation-assisted tunnelling or variable-range hoppingmodels. This could be explained by the fact that thelatter mechanisms (uctuation-assisted tunnelling or variable-range hopping) are valid for low temperatures and areinappropriate above room temperature. Kulesza et al [25]claim that these two well dened activation energies canbe discussed in terms of different conductivity mechanisms,taking into account metallic and semiconducting contributions.Further on, Cha et al [26] claim that this behaviourcan be explained by considering the conduction throughsemiconductorsemiconductor or metalmetal contacts (lowactivation energy) and semiconductormetal contacts (higheractivation energy). These models also seem to broadly explainthe experimental results presented in this work.

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Table 2. Values of the specic electrical conductivity at three representative temperatures for all piranha-treated CNT sheet composites.

(S cm 1)

SampleEpoxy concentration insoaking solution (%) T = 60 C T = + 20 C T = + 90 C

Piranha buckypaper 0 12.99Piranha composite 30 7.10 7.69 8.11

Piranha composite 40 5.85 6.02 6.21Piranha composite 50 3.21 3.24 3.58Piranha composite 60 1.92 2.04 2.17Piranha composite 70 0.41 0.44 0.47

3.3. Effect of oxidation treatment on the mechanicalbehaviour of impregnated composite CNT sheets

Previous studies have shown that modication of CNT materialwith functionalities having a relatively high oxygen contentpromotes adhesion to epoxy resins [ 24, 27]. This isanalogous to similar improvements observed in the case of

oxidative treatment of other graphitic materials such as carbonbres [28, 29]. It is thus expected that the improvementof interfacial adhesion should lead to improvements of certain aspects of the mechanical behaviour of CNTepoxycomposites as compared to the neat polymer matrix. For thisreason, we employed the immersion/soaking method describedabove to process MWCNT/epoxy nanocomposites for whichthe CNT material had undergone oxidation by KMnO 4 /H2SO4or HNO 3 [19, 21]. The prepared buckypapers were soakedin the acetone solution of resin with concentrations of 50 and60 w / w% and their properties were compared with the piranha-treated CNT sheets.

Porosimetry data presented elsewhere [ 20, 21] haverevealed that the KMnO 4-treated buckypapers possess anaverage inter-nanotube pore size ( 40 nm), much greaterthan the corresponding value of the piranha and HNO 3-treated sheets ( 12 nm). Thus it is probable that theimmersion/soaking methodology presented here is not suitablefor the production of nanocomposites derived from KMnO 4-treated buckypapers. In fact, TGA and SEM experimentsconrmed that these buckypapers contained large voids whichcould not properly have been lled by resin (gure 1(e)).The partial lling of the KMnO 4 buckypaper by the epoxyresin is the main reason for the observed poor mechanical andthermal performance of these nanocomposites (gure 3). It is

postulated, however, that processing techniques such as resin-lm-infusion used for composite production in the aerospaceindustry could be ideal for the lling and wetting of thenanotubes in these materials.

Concerning the piranha and HNO 3-treated buckypapers, itwas found that they exhibit porosity curves with a relativelynarrow distribution [ 20, 21], indicating that these CNTnetworks are well-organized and homogeneous. However, ascan be seen from TGA, the CNT content is again relativelylow in these nanocomposites. The composite from a HNO 3-treated CNT sheet soaked in 50 w / w% epoxy solution inacetone contained in effect 73% of polymer matrix andthe corresponding lm soaked in 60 w / w% epoxy solutioncontained 83% of polymer (table 1). In fact, a combinationof a polar solvent, such as acetone, with a high oxygen content

CNT network (after HNO 3 oxidation) does not seem to favourthe diffusion of epoxy components within the correspondingbuckypaper in thermodynamic terms. On the contrary,the epoxy monomer/curing agent mixture may diffuse moreefciently within the less polar piranha-treated buckypaper,thus yielding composites with relatively higher epoxy content(lower CNT fraction, table 1).

The overall mechanical behaviour of the materialsexamined in this work is presented in gure 3. Certainvery important observations can be made concerningreinforcement efciency for the nanocomposites preparedby the immersion/soaking method. The elastic modulus(gure 3(a)) of the impregnated piranha-treated CNT sheetseems to increase up to 90 w / w% polymer content asdetermined by TGA (soaking with 40 and 50 w / w% of epoxysolution) and then drops slightly. The piranha-treated CNTsheet exhibits the highest value of 3.5 GPa which is 12 timeshigher than that of the dry sheet and 100% higher than thevalue of neat resin (1.8 GPa). The corresponding values of

tensile strength against the polymer content are presented ingure 3(b). As seen, the tensile strength increases rapidlyas the amount of polymer increases. However, it lookslike the upper limit, at least for the piranha-treated sheet,is the value of the neat resin reached at 91% of polymercontent (60% epoxy in acetone). On the other hand, theoxidative treatment obtained with the HNO 3 seems to havea benecial effect on the composite tensile strength, and avalue of approximately 37 MPa that far exceeds that of theneat resin (23 MPa) is measured at 65% of polymer content(60% epoxy in acetone solution). Overall, whereas a notableincrease of the Youngs modulus to that of the neat resin is

observed in randomly oriented CNTepoxy nanocomposites,an appreciable improvement of the tensile strength has notbeen achieved [ 13]. To our knowledge, only in a recentwork of Xu et al [14] was a 55% enhancement of thecomposite tensile strength observed by infusion of polyvinylalcohol chains within a CNT membrane. Moreover, a 73%enhancement for elastomeric polyurethanes was observed byBlighe et al [18]. The values of strain to failure for thepiranha-treated composites, shown in gure 3(c), are generallybelow the value of dry buckypaper and that of the neat resinindicating that the increase of modulus by resin impregnationis achieved at the expense of composite ductility. This iscertainly not the case for the extremely brittle HNO 3-treateddry buckypaper, whereas a marked improvement of the strainto failure is obtained at high (65%) resin content within the

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(a)

(b)

(c)

Figure 3. Graphs of mechanical properties of buckypaper/epoxycomposites as a function of increasing polymer content: (a) tensilemodulus as a function of w / w polymer content, (b) tensile strengthas a function of w / w polymer content and (c) strain-to-failure as afunction of polymer content.

composite. Based on the data of gure 3, one can concludethat the mechanical behaviour of the impregnated compositewill depend on the oxidative treatment (interface quality)but also on the overall epoxy concentration. Under certaincircumstances the values of both modulus and tensile strengthof the neat resin can be exceeded.

As mentionedabove, we compared our immersion/soakingmethod with an analogous procedure described in the work of Wang et al [8], in which a piranha-treated buckypaper was im-pregnated by epoxy through repeated ltration of acetone solu-tion in the thickness direction. The main disadvantages of theltration process are the increased time of procedure, the need

to use more complicated apparatus and the non-uniform resincoverage on both sides of the buckypaper. TGA analysis (ta-ble 1) conrmed that the composite sample contains a some-what higher amount of epoxy resin compared to the soakedsample with equivalent concentration. This may well be ex-plained by the existence of an excess of epoxy resin on the topside of the composite coupon, as observed by Pham et al [13].The mechanical properties (e.g. the values of E , b and b)of the sample termed Filtration30 are comparable to those of sample soaked with 50 w / w% epoxy solution.

In recent years there has been a growing demand fortextiles and lightweight engineered bre-based structures withfully integrated functions. Buckypaper-based composites area key enabling technology component for this convergence,combining high specic strength and stiffness as well aselectrical and thermal conductivity. The tensile strength of theimpregnated CNT lms seems to far exceed the relatively lowstrength values exhibited by other conductive polymers suchas polyacetylenes [ 30]. The reasonable enhancement of bothmodulus and strength as regards the mechanical performanceof dry CNT lms indicates that these nanocompositescan be used as membranes/lms in applications whereelectrical conductivity is the desired property. The rangeof potential applications of buckypaper composite lms isquite broad. Such CNT/polymer lms could be the ideal

candidates to substitute for or complement conventionalcarbon bre/polymer composites in the fabrication of multifunctional parts in the aircraft and/or automobileindustries. Recent studies have demonstrated that analogouslaminated composites can function as lm electrodes [16],excellent electro-mechanical actuators [ 31], piezoresistivestrain sensors for structural health monitoring [ 32], lightningstrike damage mitigators in the aircraft industry [33] andelectromagnetic interference shielding components [ 34]. Thechange of focus from the quest of improving the resincharacteristics to that of improving the mechanical integrityof the buckypaper for conductivity advantages is the mainoutcome of the work presented here.

4. Conclusions

An effective and versatile approach for producing multifunc-tional carbon nanotubeepoxy composites with high loadingof the ller material was presented. A controlled nanostruc-ture of CNT/epoxy thin sheet can be achieved through impreg-nating CNT networks (buckypapers) with epoxy solution. Bychanging various parameters, such as concentration of epoxysolution, oxidation state of CNT material and impregnationprotocol, we concluded that we can actually tune the mechan-ical/electrical properties of the prepared composites. Mechan-ical measurements showed an enhancement of Youngs modu-lus and a tensile strength of about 100 and 60%, respectively.

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In addition, electrical measurements showed that the specicconductivity of the composites has comparable values to thatof the neat buckypaper which is quite remarkable for a varietyof applications such as conductive composite lms for electro-magnetic shielding, as well as for components in the automo-bile industry.

Acknowledgments

Financial support from the Marie Curie Transfer of Knowledgeprogram CNTCOMP (contract no. MTKD-CT-2005-029876)is gratefully acknowledged. We thank Drs M Labou andV Drakopoulos for their assistance with the TGA and SEMdata. Drs K Papagelis and J Parthenios are thanked for usefuldiscussions and for providing advice and assistance with theexperiments.

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